(Mangled Jet Stream on June 20th, 2014 together with cut-off upper air low threatens record-shattering storms and flood events across a multi-state region from the Dakotas to Minnesota to Iowa and Nebraska over the coming days. Image source: Earth Nullschool. Data source: NOAA.)
If you wanted an example of a Jet Stream mangled by human-caused climate change, you couldn’t find a better one than today’s tangle of upper level winds swirling over North America.
It’s a chaotic maelstrom of split flows, colliding storm tracks, blocking highs, and cut-off upper air lows. A barrel of snakes pattern that’s become ever-more-common since Arctic sea ice plummeted to staggering volume lows of nearly 80 percent less than 1979 levels at end summer of 2012. A loss that opened wide the gates for warm air to flood northward and confuse the hot-cold dividing line that drives this key weather governor.
Over the past week, we’ve seen what amounts to a mess of storms mostly locked in place. A Pacific Ocean flow squeezed between a blocking high off California and an upper level low south of Alaska drew a train of moisture trailing all the way across the Pacific into a hungry cut-off low that had stalled along the border between Canada and the US. Drifting slowly east to west, west to east, the low gorged on the synoptic moisture feed, dumping record rainfall after record rainfall over the Dakotas, Minnesota, Nebraska and Iowa.
100 Year Records Shattered
By the 16th of June, with just slightly more than half the month passed, Sioux Falls South Dakota had crushed its all-time record rainfall for any month by more than 2.5 inches. The previous record of 9.42 inches set in 1898 catapulting to a staggering 13.04 inches by early this week. And with the storm track writhing overhead the rains for the region just kept coming. By yesterday, the twin cities region in Minnesota had rocketed to its second wettest June on record amidst massive rainfall-driven landslides and region-wide preparations for Mississippi River flooding. At 10.33 inches measured rainfall so far, with storms still popping overhead, and with 11 days still remaining in the month, it appears the area may well be set to shatter the previous rainfall record of 11.67 inches set back in 1874.
(Record flooding along the Big Sioux River in Iowa and South Dakota as witnessed yesterday by Storm Chasers.)
All the massive rainfall has built up quite a pulse of flood water that is now moving down major river systems and threatens record flooding events throughout a multi-state region from the Dakotas to Minnesota to Iowa to Nebraska. Residents are being called to aid in sand bagging and other flood mitigation operations as rivers keep rising through numerous regions. According to a report today in the Christian Science Monitor:
“In Iowa, South Dakota, and Nebraska, officials are asking volunteers to build sandbag barriers and other fortifications in advance of the brunt of the storm – but politicians and emergency workers are conceding that their efforts, in some areas, may not be enough.
In South Dakota, workers have begun turning a major Interstate exchange bridge into a temporary levee. While officials there say that will mitigate the flood in many locales, Governor Dennis Daugaard (R) said he expects parts of North Sioux City, S.D., to be underwater by the end of the week.”
Storms Expected to Continue
Today a frontal boundary sweeping out from our upper air low is bringing rains to the Great Lakes and Central Plains region. Meanwhile, behind the front, instability and moisture flow beneath the low continue to result is a high risk for severe thunderstorms accompanied by strong winds, torrential downpours, hail and frequent lightning. Severe storm risks are most extreme for areas of southeastern Nebraska, western Iowa, northern and western Minnesota, and eastern North Dakota.
Already, satellite imagery shows strong storms and accompanying high cloud tops popping up over Nebraska with more likely to follow as afternoon and evening progresses.
Conditions in Context: How Climate Change Intensifies Droughts/Storms
Multiple news agencies are now gathering reports of record storm events throughout the affected multi-state region. Recording agencies and residents alike note a dramatic increase in both the frequency of record events and in their intensity.
Storm precipitation intensity is a measure of how much rain, snow, sleet or hail falls from a given storm over a given period. And what we have seen is an increasing number of record hourly rainfall events in which precipitation totals measure 1 to 2 inches or more within a 60 minute span. Such intense events rapidly overwhelm infrastructure, flood roads, and burst river banks, creating a dangerous situation that often results in numerous water rescues. And both local and national climate reports have marked a major increase in both precipitation and precipitation intensity over the past two decades for regions such as Iowa.
In the context of human-caused climate change, frequency of intense storm events is increased due to rising atmospheric moisture loading. Overall, for each 1 degree C increase in temperature, the hydrological cycle increases by about 7% in intensity. The current .8 C rise since 1880 has resulted in about a 6% increase in the rate of evaporation and of rainfall. So in regions where heat and dryness tend to take hold, the soils tend to dry out faster, tipping into drought conditions far more rapidly and seeing an overall intensification and lengthening of droughts. And in regions where storms do form, they tend to dump far more rainfall than they used to.
(Global warming intensifies thunderstorms by adding convective energy, increasing atmospheric moisture, and expanding the troposphere. As a result, thunderstorm cloud heights increase resulting in more intense rain and hail events. Image source: National Weather Service.)
Changes in the Jet Stream due to loss of sea ice in the Northern Hemisphere also tends to result in more persistent weather patterns. The Jet Stream tends to meander more, spinning off more cut off lows that linger over regions creating instability and rough weather for longer periods. High amplitude waves tend to also form as more warm air invades the higher Latitudes. In the ridges, powerful high pressures tend to dominate. And once these highs establish, they can be very difficult to move. Beneath these blocking highs, droughts proliferate due to the extreme length of dry periods and due to the intensified rate of evaporation. We see such an event now in the 15+ month long blocking high that has so greatly impacted California and the ongoing drought there.
Lastly, increasing convection and a thickening, hotter atmosphere tend to spike storm intensity. In areas where moisture and heat are both high, the explosive rate of evaporation tends to rapidly form storms with very high cloud tops. These cloud tops, now sometimes pushing 50,000 or 60,000 feet pack in more moisture and can generate very intense rainfall events over shorter periods than we are used to.
In these ways, climate change forms an ideal brew for perfect thunderstorms and perfect droughts. With temperatures expected to spike to +2 C or great anomalies over the coming century, we can look forward to extreme weather continuing to intensify with both record rainfalls and record droughts dominating with ever-increasing frequency.
Hat Tip to Colorado Bob
Hat Tip to TodaysGuestis